A number of optical measurement or analysis instruments use one or more light spots generated from coherent light sources, such as lasers or superluminescent diode (SLDs), to make optical measurements of the eye. Well-known examples of such instruments include wavefront aberrometers (e.g., Shack-Hartmann wavefront aberrometers), corneal topographers, as well as optical coherence tomographers. A new class of combined instruments is also emerging for performing comprehensive eye measurements for refractive errors and/or for carrying out diagnostic measurements and analysis for cataract procedures, including for example, intraocular lens placement and alignment.
An undesirable feature of the light sources used in some of these instruments is that the light pattern produced in the instrument is marred by speckle. Speckle is a spotty pattern with large light intensity variations. FIG. 1 illustrates an example of a speckle pattern. Speckle is caused when the layer from which the light is scattered is thinner than the coherence length of the light source. A typical SLD has a bandwidth of thirty nanometers, which corresponds to a coherence length of one hundred microns.
Speckle can cause problems with some optical measurement or analysis instruments. For example, there are two ways that speckle causes measurement errors in an instrument that employs a Shack-Hartmann wavefront sensor. One problem is that the mathematical algorithms called reconstructors that are employed by such instruments have fitting errors in data sets that contain dark regions of the speckle pattern. Another problem is “intensity coupling.” Intensity coupling may occur when a wavefront sensor is constructed such that the lenslet array is not located exactly one focal length from the pixel array. In that case, intensity variations cause shifts in the spot locations that are independent of the slope of the wavefront. These shifts cause errors in the calculated wavefront.
With the human eye, speckle is mitigated because the scattering occurs in a volume that has a thickness that is longer than the coherence length of the light source. The light penetrates into a layer of the eye and weak scatter occurs throughout the volume. As a result, when an SLD light source illuminates a human eye, the bright to dark ratio is typically about two to one.
Meanwhile, it is sometimes necessary to be able to verify correct operation and specified performance of an optical measurement instrument such as those described above in an operational setting. In many instances, this is done by operating the measurement instrument to make a measurement of a model eye whose characteristics are known. In that case, typically the optical measurement instrument injects a probe beam into a front surface of the model eye. Light scatters from the back surface of the model eye similarly to the way it does with a human eye, and some of the scattered light travels back out of the front surface and into the optical measurement instrument.
When a typical model eye is measured, however, the speckle is more severe than that typically seen when measuring a human eye. The problem is further exacerbated by the fact that the “cornea” of the model eye acts like a magnifying glass and makes the structure of the bright spots and dark regions appear large on the detector inside the optical measurement instrument. The typical speckle pattern for a model eye has a bright to dark ratio of twenty-to-one, which is about an order of magnitude greater than that of a speckle pattern for a human eye. For all these reasons, these large variations in light level cause inaccurate measurements that can in turn affect diagnosis and treatment.
Therefore, an improved model eye and systems and methods employing the same are desired.